Render archaeological cisterns using both icons

[zstadler]

The combination of man_made=cistern and historic=archaeological_site does not show the cistern icon. For example, here

As a result, many ancient cisterns in the desert are shown as archaeological sites without their other significant role.

The goal is to show both icons, similar to the old IHM raster map

[zstadler]

Analysis

The root-cause of the issue is that both man_made=cistern and historic=archaeological_site are included in the poi layer. When a combined element is detected, the layer must choose one classification or another.

Design

A solution would require creating a separate layer for one of the element types. Since several categories in the poi layer were added specifically for the IHM schema, it would make sense to move them into a new ihm_poi layer. This will also reduce the difference between the IHM and OMT schemas and reduce the cost of future merges.

Deployment

1. Implement the new ihm_poi layer
2. Modify the style files to reference the ihm_poi layer
3. Remove the redundant elements from the poi layer

[zstadler]

Potentially, imposm's mappings directive allows including a geometry multiple times.

[zstadler]

Collision avoidance for water poi labels was disabled.